The overarching goal of this project for the past decade has been to elucidate the biochemical and molecular mechanisms of V(D)J recombination. We have been fortunate to discover a number of very interesting properties of the recombinase, and the new studies proposed here capitalize on a serendipitous convergence of investigations into mechanisms of cleavage and transposition. More exciting for us as immunologists, we are poised to bring this new molecular, mechanistic understanding to bear on large biological questions of autoimmunity and host defense. Specifically, recent work in my lab challenges the prevailing models of transposition and its role in oncogenic rearrangements, and suggests a new connection between repertoire restriction and autoimmunity In the three aims of this grant we propose to: I. Test a novel release and rebind model for RAG-mediated transposition; II. Investigate the effects of target structure and sequence on transposition and hybrid joint formation; III. Construct and analyze mice bearing RAG-1 allies with conditional recombination defects.